The present invention relates to a control linkage for effecting speed and direction changes in a hydrostatic transmission by controlling the position of a swash plate of a reversible flow, variable displacement pump forming a part of the hydrostatic transmission.
It is common practice to use hydrostatic transmissions for driving the drive wheels of various vehicles including lawn and garden tractors and to employ a control linkage including a shift lever manuallly movable in opposite directions from a neutral position, wherein the swash plate is positioned so that no fluid is pumped by the pump, respectively to forward and reverse driving positions, wherein the swash plate is respectively positioned so as to cause fluid to be pumped in opposite first and second directions through the pump at rates which increase as the displacement of the lever from its neutral position increases.
These prior art control linkages are designed such that movement of each shift lever is transferred directly to the associated swash plate, so as to cause it to move in concert with the shift lever at a speed directly related to the speed of shifting of the lever. The drive speed of these transmissions, especially as normally employed in lawn and garden tractors, is quite responsive to swash plate movement. Therefore, one problem attendant with the known control linkages is that the vehicle will lurch in the event that an operator quickly moves the shift lever to effect forward and rearward drive speed increases in the transmission. This operation of the tractor is, of course, not desirable from a safety standpoint.
While "lurching" can be avoided by slowly shifting the transmission shift lever of the known control linkages, the shift levers are often operated by hand and it is undesirable, again from a safety standpoint, for an operator to have his hand preoccupied for any length of time since controlling of the steering of the tractor is more safely accomplished by using both hands.